1. Field of the Invention
The present invention relates to a pyrometallurgical process for copper smelting, and more particularly relates to a process for obtaining blister copper or white metal (Cu2S), wherein the term “white metal” is meant to cover a matte near the white metal (with very little FeS) in addition to Cu2S, by oxygen-smelting copper sulfide concentrate, or matte obtained from copper sulfide concentrate.
2. Description of the Related Art
Heretofore, copper smelting has comprised: a matte smelting process in which copper sulfide concentrate is oxygen smelted, part of the Fe in the ore is oxidized to be removed as slag, and part of the S becomes SO2, so that Cu is concentrated into matte, being a mixture of FcS and Cu2S; a subsequent white metal production process which obtains white metal (Cu2S) containing a almost no Fe, after removing Fe as slag with further oxidation of the obtained matte; and a copper production process which obtains blister copper by further oxidation of this white metal. An autogenous smelting furnace is generally used as the matte smelting furnace, while the white metal production process and the copper production process are usually carried out in a converter. The converting furnace is a batch type.
Since copper sulfide concentrate normally contains SiO2 as gangue, the matte smelting process uses iron silicate slag. The converter also normally forms iron silicate slag by the addition of silica minerals as flux.
A matte smelting furnace produces matte, in which the copper content of the matte (matte grade; MG) is normally 70% or less by weight, and charges this into the converter. A converter, being a batch type, converts the matte into white metal, and subsequently into blister copper as describes above. To increase the productivity of the whole plant, it is desired to increase the MG in a matte smelting furnace and reduce the load in a batch type converter. If the matte smelting furnace can continue oxidization until white metal is produced, the white metal production process in the converter becomes necessary. Furthermore, if it can oxidize to blister copper, the converter process itself becomes unnecessary. However, if an attempt is made to increase the oxidation degree in the matte smelting furnace, the following problems caused by iron silicate slag occur.
(1) Magnetite complications:
In iron silicate slag, the solubility of trivalent Fe is low. This causes so-called magnetite complications wherein solid magnetite is precipitated and deposited on the bottom of the furnace, and the like. To avoid this problem, in the case when MG is increased, the smelting temperature must be raised up to 1300° C. or more. However, this accelerates damage to the furnace body. Furthermore, when the copper content of the slag is increased by oxidizing part of the copper, even though iron silicate slag can produce blister copper without magnetite complications, the copper content of the slag in this case needs to be 25% or more and the yield of blister copper is considerably lowered.
(2) Oxidation and dissolution of copper:
As MG increases, the solubility of copper, as oxide, in iron silicate slag increases considerably.
(3) Concentration of impurities:
In the presence of iron silicate slag and matte or blister copper, since the solubility of oxides of As, Sb and the like into iron silicate slag is low, these impurities concentrate into the matte or the blister copper. The effect is particularly high when iron silicate slag and blister copper coexist, and this is regarded to be one of the reasons why blister copper cannot be obtained directly from copper sulfide concentrate with high impurities in the presence of iron silicate slag.
From these points, a matte smelting furnace is normally operated with approximately 65 to 70% MG as the upper limit.
Furthermore, because of similar problems, in a process of oxidizing matte into low S content of blister copper, continuous processing is regarded to be impossible in the presence of iron silicate slag, and usually a batch process using a converter is carried out. There is a report (Japanese Unexamined Patent Publication No. Sho 58-224128) describing blister copper continuously obtained from matte in the presence of iron silicate slag. In this case, however, blister copper was obtained in the presence of three phases of slag, white metal and blister copper, and it was unavoidable for the S content of the blister copper thereof to be as high as 1.5%, increasing the load of the operation in the following processes, a refining furnace, considerably.
Avoiding these problems, one of the inventors of the present invention has proposed a method to produce white metal in a matte smelting furnace in Japanese Examined Patent Publication No. Hei 5-15769. This is to remove iron in copper sulfide concentrate as calcium ferrite slag by adding lime as flux. There is an advantage in the use of calcium ferrite slag in that precipitation of magnetite is avoided and the elimination of impurities such as As, Sb or the like in the slag is higher than iron silicate slag. However, there are problems as described below.
(1) Copper sulfide concentrate normally contains some SiO2. Therefore to produce as pure a calcium ferrite slag as possible, the copper sulfide concentrate to be processed is restricted to that with a low content of SiO2 (3% or less).
(2) Even with copper sulfide concentrate with a low content of SiO2 as mentioned above, if there is a little SiO2 in the calcium ferrite slag, it worsens the viscosity and causes foaming, which renders it difficult to have a stable furnace operation. Therefore, when calcium ferrite slag is used, the content of SiO2 in the slag should be regulated to be 1% or less (about 1.7% or less by weight with respect to Fe in the slag). In the case of obtaining white metal from standard copper sulfide concentrate mainly composed of chalcopyrite with this method, the SiO2 content of the copper sulfide concentrate is restricted to 0.4% or less for practical purposes.
(3) Since the solubility of Pb into calcium ferrite slag is low, Pb is difficult to distribute into the slag, and becomes concentrated in the white metal.
(4) The amount of copper dissolving into the calcium ferrite slag as oxide is large, and the recovery percentage by concentration is low.
On the other hand, in a converter process, when matte is converted into white metal or blister copper by further oxidation, to avoid the problems caused by iron silicate slag, the process being in batches, the furnace temporarily stops blowing in the presence of white metal and slag and tilts to remove the slag, leaving only the white metal in the converter to oxidize into blister copper. This method, which has various drawbacks caused by the batch type process, makes the converter operation cumbersome.
The Mitsubishi continuous copper smelting process avoids magnetite precipitation by using calcium ferrite slag in the process of a converter (C furnace) and produces blister copper continuously from matte of approximately 65% MG. However, there are the following problems caused by calcium ferrite slag.
(1) The copper content or the slag changes continuously with respect to oxygen partial pressure, and as the S content of the blister copper is lowered, the copper content of the slag becomes higher. In practice, when the S content of the blister copper is approximately 0.5 to 1%, the Cu content of the slag is 13 to 15%, and it is not effective in terms of copper yield for the S content to be less than or equal to this.
(2) The copper content in calcium ferrite slag is mainly oxide which is chemically dissolved, and even with slow cooling, the copper recovery rate by concentration is low.
(3) As aforementioned, when the SiO2 of calcium ferrite slag reaches approximately 1 to 3%, the viscosity increases considerably and foaming occurs. Therefore, it is difficult to use matte containing iron silicate slag as raw material. When the Fe content of matte is 10%, the SiO2 allowed to mix into the matte is 0.2% or less with respect to the matte, and it is necessary to pay special attention to avoid the slag mixing into the matte produced in the matte smelting process.
(4) Since Pb solubility is low, Pb is difficult to distribute into the slag, so that it becomes concentrated in the blister copper. It is therefore difficult to produce an anode capable of electrolysis from high Pb content raw material with a conventional process.
(5) When compared at the same temperature, because its permeability in refractories is high, it cause greater erosion of refractories in the converter than silicate slag.
In regards to iron calcium silicate slag, Japanese Patent Unexamined Publication No. 2000-63963 proposes an area in which a weight ratio of CaO/(SiO2+CaO) is 0.3 to 0.6, and a weight ratio of Fe/(FeOx+SiO2+CaO) is 0.2 to 0.5. This area is determined because the slag that is separated from the area is not completely molten and compounds with a high melting point are precipitated out at normal smelting temperature up to 1350° C. in the production conditions for the while metal, the matte close to white metal or the blister copper.